It's generally nice to avoid taking complex problems and making them even more complex—but not always. There may be some real advances in clarity to be gained from incorporating our new questions about "theft" into the larger puzzle regarding Proudhon and "property." But we're going to have to proceed cautiously. Let's begin with a sort of catalog of the concepts that may or may not be in play, as we try to unpack Proudhon's infamous phrase, "property is theft," in the contents of his remarks on the commandment, "thou shalt not steal."
THEFT: We have two likely definitions of the term "theft." It may be taken in a fairly conventional sense, to mean misappropriation of already-existing "property."
Alternately, it may mean some broader category of blameworthy resource misuse, involving "holding, turning or putting aside." Perhaps we shouldn't call this "theft" at all, since presumably it is "the concept mistranslated as 'theft' (in the first sense)," except that our most immediate concern is whether or not the "theft" in "property is theft" should be understood according to a narrow definition, presupposing some form of legitimate "property," or more broadly, in a sense that may be antagonistic to all forms of "putting away." We'll think of both as possible definitions of "theft" in this particular context, and I don't think we'll have gone too far wrong.
RESOURCE RELATIONSHIPS THAT ARE NOT THEFT: With the first, narrow sense of "theft," we know that the other key concept presupposed by the definition is, in fact, "property." We may not know the details of what that "property" entails, but we know that we can't very well get to "theft" (of "property") without that other term. (The vaguer our notion of "property," the vaguer, naturally, will be our concept of "theft"—and we know that there is an extremely vague notion of "property" lurking in the background of Proudhon's work. We also know, however, that he seems to have been addressing the phrase "property is theft" to a much more clearly defined variety.)
But if "putting aside" is not blameworthy because there is a previously existing form of legitimate property that is somehow being abused, but because of some other sort of impropriety (and, yes, the two may be hard to separate), we are left to figure out what "not putting aside" is in its positive form, and what is positive about it. If, for instance, "property is theft" because the precondition for any sort of property would be an accumulation or "putting aside" that would violate the broader interpretation of the injunction against "theft," we would be left to determine just what relationship or order is being disrupted by the diversion of resources. The most obvious candidate for an equivalent of "not putting aside" is probably the "community" (communauté) that Proudhon proposes as the first form of sociability, and the thesis to the antithesis of "property," in 1840. That term has unfortunately been translated by Tucker as "communism," leading to misunderstandings and questionable "clarifications" about what Proudhon was objecting to in his criticisms of "community." I think this is a case where we're frequently been too quick to leap to the defense of labels, when a little closer attention to the relevant texts would have made it clear that Proudhon was playing a rather different game. This isn't the place for a thorough treatment of what Proudhon said about "community" in 1840, but it probably is worth noting that he characterized it in one key passage as the "spontaneous movement" of "sociability." If the antithesis of individual "holding, turning or putting aside" is a spontaneous and sociable movement, maybe we at least have the beginnings of a little more convincing thesis/antithesis relationship, a more interesting basis for that "synthesis of community and property" that Proudhon associated with "liberty" most of the ones we have proposed thus far.
PROPERTY: That leaves us with the slipperiest of Proudhon's key-terms: "property." While he was fairly explicit about his definitions at every stage, it's not clear that he was always entirely faithful to his stated definitions, and, even if he was, he made a bit of a tangle of them. As I've already noted, Proudhon acknowledged a broad, vague category of "property," within which the key terms of his 1840 analysis, "simple property" and "simple possession," can be counted as possible varieties. This is the point on which he was probably least consistent, however, since at times he did not want to call "possession" by the name of "property" while at others he did—but during the period when he did not want to do so, he still acknowledged (in the introduction to the second edition of What is Property?) that by "property" he meant "the abuse of property." He never seems to have denied the possibility of a "property" that would not be "theft" for very long at a stretch.
"Simple property," or "domain," was consistently defined as a "matter of right," and specifically of "the right of use and abuse," and it was this "property" about which Proudhon said that "property is theft" in 1840. And it was a particularly strong form of "simple or allodial property" around which Proudhon built his "new theory" of property in The Theory of Property. In that final work, it was precisely the absolute nature of "simple property," its character as "theft," that gave it its desirable character, since that theory depended on the use of property to create a space within which the individual would be sheltered from the absolute demands of other proprietors and whatever "state" or state-like institutions might exist in a free society.
"Simple possession," which Proudhon constantly lumps together with "fief," is, on the contrary, a "matter of fact." Proudhon admitted, in The Theory of Property, that he hadn't really defined "possession" in his earlier works, and he made the following remarks on the subject:
Possession, indivisible, untransferable, inalienable, pertains to the sovereign, prince, government, or collectivity, of which the tenant is more or less the dependent, feudataire or vassal. The Germans, before the invasion, the barbarians of the Middle Ages, knew only it; it is the principle of all the Slavic race, applied at this moment by the Emperor Alexander to sixty millions peasants. That possession implies in it the various rights of use, habitation, cultivation, pasture, hunting, and fishing—all the natural rights that Brissot called property according to nature; it is to a possession of that sort, but which I had not defined, that I referred in my first Memoir and in my Contradictions. That form of possession is a great step in civilization; it is better in practice than the absolute domain of the Romans, reproduced in our anarchic property, which is killing itself with fiscal crises and its own excesses. It is certain that the economist can require nothing more: there the worker is rewarded, his fruits guaranteed; all that belongs legitimately to him is protected. The theory of possession, principle of civilization of the Slavic societies, is the most honorable of that race: it makes up for the tardiness of its development and makes inexpiable the crime of the Polish nobility.
But is that the last word of civilization, and of right as well? I do not think so; one can conceive something more; the sovereignty of man is not entirely satisfied; liberty and mobility are not great enough.
(Unfortunately, Proudhon also observed, in What is Property?, that: "Possession is a right; property is against right." This isn't necessarily a contradiction of the other characterizations, but it certainly will complicate matters more than a bit, when the time comes to talk about "right" and "rights.")
The Theory of Property contains a chapter on "The Various Meanings of the Word 'Property'," which clarifies a few points in the earlier works. But most of the difficulties in Proudhon's works on property are not really terminological. Of those that are, most of the thorniest revolve around the ill-defined or undefined "possession."
If we focus, for the moment, on unpacking the possible meanings of the phrase "property is theft," we seem to have most of what we need. There are a range of potential interpretations, the least interesting of them being the one suggested by the redefinition of "property" as "abuse of property," which means that the phrase can be rendered as: The abuse of property is theft. And this in turn breaks down, depending on how we define "theft," into either "the abuse of property is the abuse of property," or, perhaps, "the abuse of property—which is based on "putting aside," which makes it theft—is theft." But if we accept the broader definition of "theft," then we've already essentially defined "simple property" as "theft." Honestly, neither formulation seems to pack quite the punch of the phrase with all of its apparent scandal and paradox intact. But we've probably become a little too attached to the scandal and paradox anyway.
What have we gained from all this complication and exploratory explication? I think we have at least clarified the sorts of questions we need to answer to move beyond the in/famous phrase, and perhaps more successfully pursue the project of that "synthesis of community and property," and to confront the thorny problem of "possession."